Colin Powell does not need more humiliation over the manifold errors in his February 2003 presentation to the UN. But yesterday a London jury brought down another section of the case he made for war - that Iraq and Osama bin Laden were supporting and directing terrorist poison cells throughout Europe, including a London ricin ring.

Yesterday's verdicts on five defendants and the dropping of charges against four others make clear there was no ricin ring. Nor did the "ricin ring" make or have ricin. Not that the government shared that news with us. Until today, the public record for the past three fear-inducing years has been that ricin was found in the Wood Green flat occupied by some of yesterday's acquitted defendants. It wasn't.

The third plank of the al-Qaida-Iraq poison theory was the link between what Powell labelled the "UK poison cell" and training camps in Afghanistan. The evidence the government wanted to use to connect the defendants to Afghanistan and al-Qaida was never put to the jury. That was because last autumn a trial within a trial was secretly taking place. This was a private contest between a group of scientists from the Porton Down military research centre and myself. The issue was: where had the information on poisons and chemicals come from?

The information - five pages in Arabic, containing amateur instructions for making ricin, cyanide and botulinum, and a list of chemicals used in explosives - was at the heart of the case. The notes had been made by Kamel Bourgass, the sole convicted defendant. His co-defendants believed that he had copied the information from the internet. The prosecution claimed it had come from Afghanistan.

I was asked to look for the original source on the internet. This meant exploring Islamist websites that publish Bin Laden and his sympathisers, and plumbing the most prolific source of information on how to do harm: the writings of the American survivalist right and the gun lobby.

The experience of being an expert witness on these issues has made me feel a great deal safer on the streets of London. These were the internal documents of the supposed al-Qaida cell planning the "big one" in Britain. But the recipes were untested and unoriginal, borrowed from US sources. Moreover, ricin is not a weapon of mass destruction. It is a poison which has only ever been used for one-on-one killings and attempted killings.

If this was the measure of the destructive wrath that Bin Laden's followers were about to wreak on London, it was impotent. Yet it was the discovery of a copy of Bourgass's notes in Thetford in 2002 that inspired the wave of horror stories and government announcements and preparations for poison gas attacks.

It is true that when the team from Porton Down entered the Wood Green flat in January 2003, their field equipment registered the presence of ricin. But these were high sensitivity field detectors, for use where a false negative result could be fatal. A few days later in the lab, the team found that there was no ricin. But when this result was passed to London, the message reportedly said the opposite.

The planned government case on links to Afghanistan was based only on papers that a freelance journalist working for the Times had scooped up after the US invasion of Kabul. Some were in Arabic, some in Russian. They were far more detailed than Bourgass's notes. Nevertheless, claimed a government scientist, they showed a "common origin and progression" in the methods, thus linking the London group of north Africans to Afghanistan and Bin Laden.

The weakness of that case was that neither the scientist nor the intelligence services had examined any other documents that could have been the source. We were told Porton Down and its intelligence advisers had never previously heard of the "Mujahideen Poisons Handbook, containing recipes for ricin and much more". The document, written by veterans of the 1980s Afghan war, has been on the net since 1998.

All the information roads led west, not to Kabul but to California and the US midwest. The recipes for ricin now seen on the internet were invented 20 years ago by survivalist Kurt Saxon. He advertises videos and books on the internet. Before the ricin ring trial started, I phoned him in Arizona. For $110, he sent me a fistful of CDs and videos on how to make bombs, missiles, booby traps - and ricin. We handed a copy of the ricin video to the police.

When, in October, I showed that the chemical lists found in London were an exact copy of pages on an internet site in Palo Alto, California, the prosecution gave up on the Kabul and al-Qaida link claims. But it seems this information was not shared with the then home secretary, David Blunkett, who was still whipping up fear two weeks later. "Al-Qaida and the international network is seen to be, and will be demonstrated through the courts over months to come, actually on our doorstep and threatening our lives," he said on November 14.

The most ironic twist was an attempt to introduce an "al-Qaida manual" into the case. The manual - called the Manual of the Afghan Jihad - had been found on a raid in Manchester in 2000. It was given to the FBI to produce in the 2001 New York trial for the first attack on the World Trade Centre. But it wasn't an al-Qaida manual. The name was invented by the US department of justice in 2001, and the contents were rushed on to the net to aid a presentation to the Senate by the then attorney general, John Ashcroft, supporting the US Patriot Act.

To show that the Jihad manual was written in the 1980s and the period of the US-supported war against the Soviet occupation was easy. The ricin recipe it contained was a direct translation from a 1988 US book called the Poisoner's Handbook, by Maxwell Hutchkinson.

We have all been victims of this mass deception. I do not doubt that Bourgass would have contemplated causing harm if he was competent to do so. But he was an Islamist yobbo on his own, not an Al Qaida-trained superterrorist. An Asbo might be appropriate.

· Duncan Campbell is an investigative writer and a scientific expert witness on computers and telecommunications. He is author of War Plan UK and is not the Guardian journalist of the same name